After the Last Glacial Maximum, solar radiation at high latitudes remains weak. The expected warming fails to materialise, and the Laurentide and Scandinavian ice sheets retain their mass.
The moment when, in the actual timeline, the Holocene began comes and goes without any thawing. The ice caps remain in place, sea levels stay low, and the Bering land bridge remains passable, dry land.
Without the fertile, warm conditions of a Holocene, grain agriculture such as that in the Fertile Crescent does not emerge. For centuries, people remain hunters, fishers, and gatherers on a large scale.
Despite the ongoing ice age climate, mammoths, woolly rhinoceroses, and cave lions still disappear, this time mainly due to human hunting pressure on a shrinking but still extensive habitat.
Human civilizations develop along the broad, exposed continental shelves rather than around flooded river plains. Trade networks follow the coastlines of a world with a much lower sea level.
In the present, people still live in a glacial era. Civilizations have adapted to permafrost and short growing seasons, with technology focused on warmth, storage, and survival in the cold rather than on expansion into warmer, uninhabited continents.
Where the original timeline predicts a new future ice age fifty thousand years from now, this world at that point still finds itself in the same unbroken glacial period that began at the Last Glacial Maximum.
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Where in actual history the Earth's orbit gradually brought more summer sun to the north, this now barely happens. The previous winter's snow survives the summer in more and more places. Geologists studying this world would see no retreating glacier margins but a fragile balance tipping the wrong way. The mammoth steppe remains intact, vast and cold, from Ireland to Siberia.